1. Field of the Invention
The present invention relates to engine control strategies for engines and, more particularly, control methods for diesel engines having a diesel particulate filter (DPF).
2. Background Art
Diesel particulate filters (DPFs) are typically used in diesel engines. These filters store particulates generated by engine combustion. When the filter is raised above a predetermined temperature, the retained particles, which are mostly in the form of carbon, are burned off. After a majority of the particles are burned off, the filter can again retain particles generated by the engine.
There are various methods to raise and sustain particulate filter temperature to allow the above-described regeneration. For example, a burner can be used to burn fuel and thereby generate heat. Further, the burner can be controlled based on engine speed and load and temperature as described in U.S. Pat. No. 5,044,158. It is also known that the rate of regeneration is a function of temperature. For example, as described in SAE Paper No. 830180, the rate of regeneration increases with increasing temperature, following an Arrhenius equation.
The inventors herein have recognized that high regeneration temperatures can actually result in a lower integrated fuel economy penalty; and, therefore, it may be desirable to use temperature increasing measures to increase the temperature of the DPF to a level at which the particles (soot) burn rather rapidly. In particular, if enough oxygen is available and the exhaust flow does not absorb too much heat, the reaction may become self-sustaining, i.e., it will continue even after the temperature increasing measures have been deactivated. In other words, it may become unnecessary to use energy, for example, in a burner, to maintain high DPF temperatures sufficient to maintain regeneration.
However, the inventors herein have also recognized a disadvantage with the above approach. In particular, while self-sustained regeneration can decrease a fuel economy penalty, it can cause other disadvantages. In particular, if the reaction rate is too high, the combustion of soot on the filter may result in excessive temperatures that degrade the DPF.
Finally, the inventors have realized that relying on exhaust air flow alone to carry heat away from the DPF may not provide sufficient cooling. In particular, it may not be clear ahead of time if a certain operating condition will allow enough air flow to cool the DPF.